Reducing acetals during ethanol separation process

ABSTRACT

To reduce acetal concentrations when separating ethanol from a crude product in one or more distillation column, at least one of the columns is operated at a higher pressure to increase the equilibrium constant that favors hydrolysis of the acetal. The crude product may comprise ethanol, acetaldehyde, water and one or more acetals, such as diethyl acetal. The acetal concentration may be reduced thus reducing the need to separate acetal from the crude product.

FIELD OF THE INVENTION

The present invention relates generally to processes for producingethanol and, in particular, to processes for separating ethanol from acrude product in one or more distillation columns that are operated atan increased pressure to reduce acetal concentration.

BACKGROUND OF THE INVENTION

Ethanol for industrial use is conventionally produced from petrochemicalfeed stocks, such as oil, natural gas, or coal, from feed stockintermediates, such as syngas, or from starchy materials or cellulosematerials, such as corn or sugar cane. Conventional methods forproducing ethanol from petrochemical feed stocks, as well as fromcellulose materials, include the acid-catalyzed hydration of ethylene,methanol homologation, direct alcohol synthesis, and Fischer-Tropschsynthesis. Instability in petrochemical feed stock prices contributes tofluctuations in the cost of conventionally produced ethanol, making theneed for alternative sources of ethanol production all the greater whenfeed stock prices rise. Starchy materials, as well as cellulosematerial, are converted to ethanol by fermentation. However,fermentation is typically used for consumer production of ethanol, whichis suitable for fuels or human consumption. In addition, fermentation ofstarchy or cellulose materials competes with food sources and placesrestraints on the amount of ethanol that can be produced for industrialuse.

Ethanol production via the reduction of alkanoic acids and/or othercarbonyl group-containing compounds has been widely studied, and avariety of combinations of catalysts, supports, and operating conditionshave been mentioned in the literature. During the reduction of alkanoicacid, e.g., acetic acid, other compounds are formed with ethanol or areformed in side reactions. These impurities limit the production andrecovery of ethanol from such reaction mixtures. For example, duringhydrogenation, esters are produced that together with ethanol and/orwater form azeotropes, which are difficult to separate. In addition whenconversion is incomplete, unreacted acid remains in the crude ethanolmixture, which must be removed to recover ethanol.

EP02060553 describes a process for converting hydrocarbons to ethanolinvolving converting the hydrocarbons to ethanoic acid and hydrogenatingthe ethanoic acid to ethanol. The stream from the hydrogenation reactoris separated to obtain an ethanol product and a stream of acetic acidand ethyl acetate, which is recycled to the hydrogenation reactor.

U.S. Pat. No. 3,102,150 discloses hydrogenating aldehydes to alcoholsusing a hydrogenation catalyst and a cation exchange resin to reduceformation of acetals.

The need remains for improved processes for recovering ethanol from acrude product obtained by reducing alkanoic acids, such as acetic acid,and/or other carbonyl group-containing compounds.

SUMMARY OF THE INVENTION

In a first embodiment, the present invention is directed to a method forproducing ethanol comprising hydrogenating alkanoic acid and/or estersthereof in a reactor in the presence of a catalyst to form a crudeethanol mixture comprising acetaldehyde, ethanol, water, and diethylacetal; and separating the crude ethanol mixture in the one or morecolumns to recover an ethanol product comprises less than 1 wt. %diethyl acetal, wherein at least one column is operated aboveatmospheric pressure. In one embodiment, the ethanol product maycomprise from 0.0001 to 0.01 wt. % diethyl acetal.

In a second embodiment, the present invention is directed to a methodfor producing ethanol comprising hydrogenating alkanoic acid and/oresters thereof in a reactor in the presence of a catalyst to form acrude ethanol mixture, and separating a portion of the crude ethanolmixture in a first distillation column to yield a first residuecomprising alkanoic acid, and a first distillate comprising ethanol, andacetaldehyde. A portion of the first distillate is further separated ina second distillation column operated above atmospheric pressure toyield a second residue comprising ethanol, and a second distillatecomprising acetaldehyde. The second residues comprises less than 1 wt. %diethyl acetal.

In a third embodiment, the present invention is directed to a method forproducing ethanol comprising providing a crude ethanol mixturecomprising acetic acid, acetaldehyde, ethanol, water, and diethylacetal; separating a portion of the crude ethanol mixture in a firstdistillation column to yield a first residue comprising acetic acid, anda first distillate comprising ethanol, and acetaldehyde; and separatinga portion of the first distillate in a second distillation columnoperated above atmospheric pressure to yield a second residue comprisingethanol, and a second distillate comprising acetaldehyde, wherein thesecond residue comprises less than 1 wt. % diethyl acetal.

In a fourth embodiment, the present invention is directed to a methodfor producing ethanol comprising hydrogenating alkanoic acid and/oresters thereof in a reactor in the presence of a catalyst to form acrude ethanol mixture; separating a portion of the crude ethanol mixturein a first distillation column to yield a first residue comprisingalkanoic acid, and a first distillate comprising ethanol, water, ethylacetate, and acetaldehyde; separating a portion of the first distillatein a second distillation column operated above atmospheric pressure toyield a second residue comprising ethanol and water, and a seconddistillate comprising ethyl acetate and acetaldehyde; and separating aportion of the second residue in a third distillation column operated toyield a third residue comprising water, and a third distillatecomprising ethanol, wherein the third distillate comprises less than 1wt. % diethyl acetal.

In a fifth embodiment, the present invention is directed to a method forproducing ethanol comprising providing a crude ethanol mixturecomprising acetic acid, acetaldehyde, ethanol, water, and diethylacetal; separating a portion of the crude ethanol mixture in a firstdistillation column to yield a first residue comprising acetic acid, anda first distillate comprising ethanol, water, ethyl acetate, andacetaldehyde; separating a portion of the first distillate in a seconddistillation column operated above atmospheric pressure to yield asecond residue comprising ethanol and water, and a second distillatecomprising ethyl acetate and acetaldehyde; and separating a portion ofthe second residue in a third distillation column operated to yield athird residue comprising water, and a third distillate comprisingethanol, wherein the third distillate comprises less than 1 wt. %diethyl acetal.

In another embodiment, the present invention is directed to a method forproducing ethanol comprising hydrogenating alkanoic acid and/or estersthereof in a reactor in the presence of a catalyst to form a crudeethanol mixture; separating a portion of the crude ethanol mixture inthe first distillation column to yield a first residue comprising asubstantial portion of the water fed to the distillation column, and afirst distillate comprising ethanol, acetaldehyde, and water; removingwater from the first distillate to yield an ethanol mixture stream; andseparating a portion of the ethanol mixture stream in a seconddistillation column operated above atmospheric pressure to yield asecond residue comprising ethanol, and a second distillate comprisingacetaldehyde, wherein the second residue comprises less than 1 wt. %diethyl acetal.

In yet another embodiment, the present invention is directed to a methodfor producing ethanol comprising hydrogenating alkanoic acid and/oresters thereof in a reactor in the presence of a catalyst to form acrude ethanol mixture; separating a portion of the crude ethanol mixturein the first distillation column to yield a first residue comprising asubstantial portion of the water fed to the distillation column, and afirst distillate comprising ethanol, ethyl acetate, acetaldehyde, andwater; removing water from the first distillate to yield an ethanolmixture stream; separating a first portion of the ethanol mixture streamin a second distillation column operated above atmospheric pressure toyield a second residue comprising ethanol, and a second distillatecomprising ethyl acetate and acetaldehyde, wherein the second residuecomprises less than 1 wt. % diethyl acetal; separating a second portionof the ethanol mixture stream in a third distillation column operated apressure from 0.1 to 100 kPa to yield a third residue comprisingethanol, and a third distillate comprising ethyl acetate andacetaldehyde, wherein the third residue comprises less than 1 wt. %ethyl acetate; and recovering an ethanol product from the second residueand/or third residue.

In yet another embodiment, the present invention is directed to a methodfor producing ethanol comprising hydrogenating alkanoic acid and/oresters thereof in a reactor in the presence of a catalyst to form acrude ethanol mixture comprising ethanol, water, ethyl acetate,acetaldehyde, and diethyl acetal; separating a portion of the crudeethanol mixture in the first distillation column to yield a firstresidue comprising a substantial portion of the water fed to thedistillation column, and a first distillate comprising ethanol, ethylacetate, acetaldehyde, and water; removing water from the firstdistillate to yield an ethanol mixture stream; separating a portion ofthe ethanol mixture stream in a second distillation column operatedabove atmospheric pressure to yield a second residue comprising ethanol,and a second distillate comprising ethanol, ethyl acetate andacetaldehyde, wherein the second residue comprises less than 1 wt. %diethyl acetal; separating a portion of the second distillate in a thirddistillation column operated a pressure from 0.1 to 100 kPa to yield athird residue comprising ethanol, and a third distillate comprisingethyl acetate and acetaldehyde, wherein the third residue comprises lessthan 1 wt. % ethyl acetate; and recovering an ethanol product from thesecond residue and/or third residue.

In yet another embodiment, the present invention is directed to a methodfor producing ethanol comprising hydrogenating alkanoic acid and/oresters thereof in a reactor in the presence of a catalyst to form acrude ethanol mixture comprising ethanol, water, ethyl acetate,acetaldehyde, and diethyl acetal; separating a portion of the crudeethanol mixture in the first distillation column to yield a firstresidue comprising a substantial portion of the water fed to the firstdistillation column, and a first distillate comprising ethanol, ethylacetate, acetaldehyde, and water; removing water from the firstdistillate to yield an ethanol mixture stream; separating a portion ofthe ethanol mixture stream in a second distillation column operated apressure from 0.1 to 100 kPa to yield a second residue comprisingethanol, and a second distillate comprising ethyl acetate andacetaldehyde, wherein the second residue comprises less than 1 wt. %ethyl acetate; separating a portion of the second distillate in a thirddistillation column operated above atmospheric pressure to yield a thirddistillate comprising acetaldehyde, and a third residue comprisingethanol, wherein the third residue comprises less than 1 wt. % diethylacetal; and recovering an ethanol product from the second residue and/orthird residue.

In yet another embodiment, the present invention is directed to a methodfor producing ethanol comprising providing a crude ethanol mixturecomprising acetic acid, water, acetaldehyde, ethanol, and diethylacetal; separating a portion of the crude ethanol mixture in the firstdistillation column to yield a first residue comprising a substantialportion of the water fed to the distillation column, and a firstdistillate comprising ethanol, acetaldehyde, and water; removing waterfrom the first distillate to yield an ethanol mixture stream; andseparating a portion of the ethanol mixture stream in a seconddistillation column operated above atmospheric pressure to yield asecond residue comprising ethanol, and a second distillate comprisingacetaldehyde, wherein the second residue comprises less than 1 wt. %diethyl acetal.

In yet another embodiment, the present invention is directed to a methodfor producing ethanol comprising hydrogenating alkanoic acid and/oresters thereof in a reactor in the presence of a catalyst to form acrude ethanol mixture; separating a portion of the crude ethanol mixturein a first distillation column operated above atmospheric pressure toyield a first residue comprising alkanoic acid, and a first distillatecomprising ethanol, and acetaldehyde; and separating a portion of thefirst distillate in a second distillation column to yield a secondresidue comprising ethanol, and a second distillate comprisingacetaldehyde, wherein the second residue comprises less than 1 wt. %diethyl acetal.

In yet another embodiment, the present invention is directed to a methodfor producing ethanol comprising providing a crude ethanol mixturecomprising acetic acid, acetaldehyde, ethanol, and diethyl acetal;separating a portion of the crude ethanol mixture in a firstdistillation column operated above atmospheric pressure to yield a firstresidue comprising acetic acid, and a first distillate comprisingethanol, and acetaldehyde; and separating a portion of the firstdistillate in a second distillation column to yield a second residuecomprising ethanol, and a second distillate comprising acetaldehyde,wherein the second residue comprises less than 1 wt. % diethyl acetal.

In yet another embodiment, the present invention is directed to a methodfor producing ethanol comprising hydrogenating alkanoic acid and/oresters thereof in a reactor in the presence of a catalyst to form acrude ethanol mixture; separating a portion of the crude ethanol mixturein a first distillation column operated above atmospheric pressure toyield a first residue comprising ethanol, and water, and a firstdistillate comprising ethyl acetate, and acetaldehyde; and separating aportion of the first residue in a second distillation column to yield asecond residue comprising water, and a second distillate comprisingethanol, wherein the second distillate comprises less than 1 wt. %diethyl acetal.

In yet another embodiment, the present invention is directed to a methodfor producing ethanol comprising providing a crude ethanol mixturecomprising acetic acid, acetaldehyde, ethyl acetate, ethanol, anddiethyl acetal; separating a portion of the crude ethanol mixture in afirst distillation column operated above atmospheric pressure to yield afirst residue comprising acetic acid, ethanol, and water, and a firstdistillate comprising ethyl acetate, and acetaldehyde; and separating aportion of the first residue in a second distillation column to yield asecond residue comprising water and acetic acid, and a second distillatecomprising ethanol, wherein the second distillate comprises less than 1wt. % diethyl acetal.

In yet another embodiment, the present invention is directed to a methodfor producing ethanol comprising hydrogenating alkanoic acid and/oresters thereof in a reactor in the presence of a catalyst to form acrude ethanol mixture comprising alkanoic acid, water, acetaldehyde,ethanol, and diethyl acetal; and separating the crude ethanol mixture inthe one or more columns to recover an ethanol product comprises lessthan 1 wt. % diethyl acetal, wherein at least one column has a strippingsection that comprises at least 40 stages.

In yet another embodiment, the present invention is directed to a methodfor producing ethanol comprising hydrogenating alkanoic acid and/oresters thereof in a reactor in the presence of a catalyst to form acrude ethanol mixture; separating a portion of the crude ethanol mixturein a first distillation column to yield a first residue comprisingalkanoic acid, and a first distillate comprising ethanol, andacetaldehyde; and separating a portion of the first distillate in asecond distillation column having a stripping section that comprises atleast 40 stages to yield a second residue comprising ethanol, and asecond distillate comprising acetaldehyde, wherein the second residuecomprises less than 1 wt. % diethyl acetal.

In yet another embodiment, the present invention is directed to a methodfor producing ethanol comprising hydrogenating alkanoic acid and/oresters thereof in a reactor in the presence of a catalyst to form acrude ethanol mixture; separating a portion of the crude ethanol mixturein a first distillation column to yield a first residue comprisingalkanoic acid, and a first distillate comprising ethanol, water, ethylacetate, and acetaldehyde; separating a portion of the first distillatein a second distillation column having a stripping section thatcomprises at least 40 stages to yield a second residue comprisingethanol and water, and a second distillate comprising ethyl acetate andacetaldehyde; and separating a portion of the second residue in a thirddistillation column operated to yield a third residue comprising water,and a third distillate comprising ethanol, wherein the third distillatecomprises less than 1 wt. % diethyl acetal.

In yet another embodiment, the present invention is directed to a methodfor producing ethanol comprising hydrogenating alkanoic acid and/oresters thereof in a reactor in the presence of a catalyst to form acrude ethanol mixture; separating a portion of the crude ethanol mixturein the first distillation column to yield a first residue comprising asubstantial portion of the water fed to the distillation column, and afirst distillate comprising ethanol, acetaldehyde, and water; removingwater from the first distillate to yield an ethanol mixture stream; andseparating a portion of the ethanol mixture stream in a seconddistillation column having a stripping section that comprises at least40 stages to yield a second residue comprising ethanol, and a seconddistillate comprising acetaldehyde, wherein the second residue comprisesless than 1 wt. % diethyl acetal.

In yet another embodiment, the present invention is directed to a methodfor producing ethanol comprising hydrogenating alkanoic acid and/oresters thereof in a reactor in the presence of a catalyst to form acrude ethanol mixture; separating a portion of the crude ethanol mixturein a first distillation column having a stripping section that comprisesat least 40 stages to yield a first residue comprising ethanol, andwater, and a first distillate comprising ethyl acetate, andacetaldehyde; and separating a portion of the first residue in a seconddistillation column to yield a second residue comprising water, and asecond distillate comprising ethanol, wherein the second distillatecomprises less than 1 wt. % diethyl acetal.

In yet another embodiment, the present invention is directed to a methodfor producing ethanol comprising hydrogenating alkanoic acid and/oresters thereof in a reactor in the presence of a catalyst to form acrude ethanol mixture; separating a portion of the crude ethanol mixturein a first distillation column to yield a first residue comprisingalkanoic acid, and a first distillate comprising ethanol, water, ethylacetate, and acetaldehyde; separating a portion of the first distillatein a second distillation column having a stripping section thatcomprises at least 40 stages to yield a second residue comprisingethanol and water, and a second distillate comprising ethyl acetate andacetaldehyde; and separating a portion of the second residue in a thirddistillation column operated to yield a third residue comprising water,and a third distillate comprising ethanol, wherein the third distillatecomprises less than 1 wt. % diethyl acetal.

In yet another embodiment, the present invention is directed to a methodfor producing ethanol comprising hydrogenating alkanoic acid and/oresters thereof in a reactor in the presence of a catalyst to form acrude ethanol mixture; separating a portion of the crude ethanol mixturein the first distillation column to yield a first residue comprising asubstantial portion of the water fed to the distillation column, and afirst distillate comprising ethanol, acetaldehyde, and water; removingwater from the first distillate to yield an ethanol mixture stream; andseparating a portion of the ethanol mixture stream in a seconddistillation column having a stripping section that comprises at least40 stages to yield a second residue comprising ethanol, and a seconddistillate comprising acetaldehyde, wherein the second residue comprisesless than 1 wt. % diethyl acetal.

In yet another embodiment, the present invention is directed to a methodfor producing ethanol comprising hydrogenating alkanoic acid and/oresters thereof in a reactor in the presence of a catalyst to form acrude ethanol mixture; separating a portion of the crude ethanol mixturein a first distillation column having a stripping section that comprisesat least 40 stages to yield a first residue comprising ethanol, andwater, and a first distillate comprising ethyl acetate, andacetaldehyde; and separating a portion of the first residue in a seconddistillation column to yield a second residue comprising water, and asecond distillate comprising ethanol, wherein the second distillatecomprises less than 1 wt. % diethyl acetal.

BRIEF DESCRIPTION OF DRAWINGS

The invention is described in detail below with reference to theappended drawings, wherein like numerals designate similar parts.

FIG. 1 is a schematic diagram of a hydrogenation process having fourcolumns in accordance with an embodiment of the present invention.

FIG. 2 is a schematic diagram of another hydrogenation process havingtwo columns with an intervening water separation in accordance with anembodiment of the present invention.

FIG. 3 is a schematic diagram having the intervening water separation ofFIG. 2 with parallel high and low pressure columns in accordance with anembodiment of the present invention.

FIGS. 4 and 5 are schematic diagrams having the intervening waterseparation of FIG. 2 with high and low pressure columns in series inaccordance with an embodiment of the present invention.

FIG. 6 is a schematic diagram of another hydrogenation process havingtwo columns in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION Introduction

The present invention relates to processes for recovering the ethanolfrom a crude mixture obtained by hydrogenating alkanoic acids and/oresters thereof. During hydrogenation several other organics may beformed along with ethanol in the crude mixture. The organics may also beformed during ethanol recovery and thus further reduce yields ofethanol. In particular, aldehydes may be present in the crude mixtureand acetalization of the aldehyde may produce acetals. When acetic acidis hydrogenated to ethanol, acetaldehyde may be formed as anintermediate. The acetalization of acetaldehyde (AcH) to diethyl acetal(DEA) is an equilibrium reaction as shown:

${Keq} = \frac{\lbrack{AcH}\rbrack\lbrack{EtOH}\rbrack}{\lbrack{DEA}\rbrack\lbrack {H_{2}O} \rbrack}$

Due to the relatively large ethanol concentration compared toacetaldehyde and/or diethyl acetal concentrations in the crude mixture,the equilibrium may favor acetalization. Further, the reaction mixturemay contain acetic acid, and the acetalization reaction may be furthercatalyzed in the presence of mineral acids or carboxylic acids. Thus,diethyl acetal concentrations may increase during the recovery ofethanol from the crude mixture. Diethyl acetal may be difficult toremove from ethanol, because diethyl acetal is a high boiling pointorganic relative to other organics formed during hydrogenation. Furtherapplications for ethanol in as an ethanol industrial and a fuel gradeethanol may require reduced concentrations of diethyl acetal, e.g., lessthan 1 wt. % diethyl acetal, less than 0.1 wt. % diethyl acetal or lessthan 0.01 wt. % diethyl acetal. In terms of ranges, the diethyl acetalmay be from 0.0001 to 1 wt. %, e.g., from 0.0001 to 0.1 wt. % or from0.0001 to 0.01 wt. %.

The present invention provides processes for reducing acetalconcentrations during the recovery of ethanol. Without being bound bytheory, the present invention may enhance the equilibrium to favorhydrolysis of acetal or to reduce the acetalization towards theformation of acetal. In one embodiment, from 10 to 75% of the acetal maybe decomposed in column at increased pressure, e.g., from 15 to 60% ormore preferably from 20 to 40%.

In one embodiment, acetal concentrations may be decreased by separatingthe crude mixture, or a derivative stream thereof, in a distillationcolumn that is operated at an increased pressure. In separating ethanolfrom the crude mixture, there may be several distillation columns andacetal concentration may be reduced by operating at least one of theseveral distillation columns at an increased pressure. The increasedpressure in the column may reduce the acetal concentration. The pressuremay increase depending on the concentration of acetal concentration fedto the column. In one embodiment, the increased pressure is greater thanatmospheric pressure, e.g., from 101 kPa to 5,000 kPa, e.g., from 120kPa to 4,000 kPa, or from 150 kPa to 3,000 kPa.

In some embodiments, one of the columns in the separation may beoperated at a higher pressure than the other columns to further enhanceacetal hydrolysis within that column.

As ethanol is recovered from the crude mixture, the acetal may form inany of the columns, thus it may be necessary to operate the last columnin the separation process at an increased pressure. The remainingcolumns may be operated at any pressure as needed to further separateethanol. For example, low pressure or vacuum conditions may enhanceseparation of ethyl acetate and ethanol.

In another embodiment, acetal concentration may be decreased byseparating ethanol from the crude mixture, or derivative stream thereof,in a distillation column that has a stripping section of at least 40stages, e.g., at least 50 stages or at least 60 stages. Preferably,ethanol is recovered in the residue of such a column. The distillationcolumn having this stripping section may also be operated at anincreased pressure to further reduce acetal concentrations.

In addition to diethyl acetal, embodiments of the present invention mayalso hydrolyze other acetals, such as those selected from the groupconsisting of ethyl propyl acetal, ethyl butyl acetal, dimethyl acetal,methyl ethyl acetal, and hemiacetals and mixtures thereof.

The process of the present invention may be used with any hydrogenationprocess for producing ethanol. The materials, catalysts, reactionconditions, and separation processes that may be used in thehydrogenation of acetic acid are described further below.

The raw materials, acetic acid and hydrogen, fed to the reactor used inconnection with the process of this invention may be derived from anysuitable source including natural gas, petroleum, coal, biomass, and soforth. As examples, acetic acid may be produced via methanolcarbonylation, acetaldehyde oxidation, ethylene oxidation, oxidativefermentation, and anaerobic fermentation. Methanol carbonylationprocesses suitable for production of acetic acid are described in U.S.Pat. Nos. 7,208,624; 7,115,772; 7,005,541; 6,657,078; 6,627,770;6,143,930; 5,599,976; 5,144,068; 5,026,908; 5,001,259; and 4,994,608,the entire disclosures of which are incorporated herein by reference.Optionally, the production of ethanol may be integrated with suchmethanol carbonylation processes.

As petroleum and natural gas prices fluctuate becoming either more orless expensive, methods for producing acetic acid and intermediates suchas methanol and carbon monoxide from alternate carbon sources have drawnincreasing interest. In particular, when petroleum is relativelyexpensive, it may become advantageous to produce acetic acid fromsynthesis gas (“syngas”) that is derived from more available carbonsources. U.S. Pat. No. 6,232,352, the entirety of which is incorporatedherein by reference, for example, teaches a method of retrofitting amethanol plant for the manufacture of acetic acid. By retrofitting amethanol plant, the large capital costs associated with CO generationfor a new acetic acid plant are significantly reduced or largelyeliminated. All or part of the syngas is diverted from the methanolsynthesis loop and supplied to a separator unit to recover CO, which isthen used to produce acetic acid. In a similar manner, hydrogen for thehydrogenation step may be supplied from syngas.

In some embodiments, some or all of the raw materials for theabove-described acetic acid hydrogenation process may be derivedpartially or entirely from syngas. For example, the acetic acid may beformed from methanol and carbon monoxide, both of which may be derivedfrom syngas. The syngas may be formed by partial oxidation reforming orsteam reforming, and the carbon monoxide may be separated from syngas.Similarly, hydrogen that is used in the step of hydrogenating the aceticacid to form the crude ethanol mixture may be separated from syngas. Thesyngas, in turn, may be derived from variety of carbon sources. Thecarbon source, for example, may be selected from the group consisting ofnatural gas, oil, petroleum, coal, biomass, and combinations thereof.Syngas or hydrogen may also be obtained from bio-derived methane gas,such as bio-derived methane gas produced by landfills or agriculturalwaste.

In another embodiment, the acetic acid used in the hydrogenation stepmay be formed from the fermentation of biomass. The fermentation processpreferably utilizes an acetogenic process or a homoacetogenicmicroorganism to ferment sugars to acetic acid producing little, if any,carbon dioxide as a by-product. The carbon efficiency for thefermentation process preferably is greater than 70%, greater than 80% orgreater than 90% as compared to conventional yeast processing, whichtypically has a carbon efficiency of about 67%. Optionally, themicroorganism employed in the fermentation process is of a genusselected from the group consisting of Clostridium, Lactobacillus,Moorella, Thermoanaerobacter, Propionibacterium, Propionispera,Anaerobiospirillum, and Bacteriodes, and in particular, species selectedfrom the group consisting of Clostridium formicoaceticum, Clostridiumbutyricum, Moorella thermoacetica, Thermoanaerobacter kivui,Lactobacillus delbrukii, Propionibacterium acidipropionici,Propionispera arboris, Anaerobiospirillum succinicproducens, Bacteriodesamylophilus and Bacteriodes ruminicola. Optionally in this process, allor a portion of the unfermented residue from the biomass, e.g., lignans,may be gasified to form hydrogen that may be used in the hydrogenationstep of the present invention. Exemplary fermentation processes forforming acetic acid are disclosed in U.S. Pat. Nos. 6,509,180;6,927,048; 7,074,603; 7,507,562; 7,351,559; 7,601,865; 7,682,812; and7,888,082, the entireties of which are incorporated herein by reference.See also U.S. Pub. Nos. 2008/0193989 and 2009/0281354, the entireties ofwhich are incorporated herein by reference.

Examples of biomass include, but are not limited to, agriculturalwastes, forest products, grasses, and other cellulosic material, timberharvesting residues, softwood chips, hardwood chips, tree branches, treestumps, leaves, bark, sawdust, off-spec paper pulp, corn, corn stover,wheat straw, rice straw, sugarcane bagasse, switchgrass, miscanthus,animal manure, municipal garbage, municipal sewage, commercial waste,grape pumice, almond shells, pecan shells, coconut shells, coffeegrounds, grass pellets, hay pellets, wood pellets, cardboard, paper,plastic, and cloth. See, e.g., U.S. Pat. No. 7,884,253, the entirety ofwhich is incorporated herein by reference. Another biomass source isblack liquor, a thick, dark liquid that is a byproduct of the Kraftprocess for transforming wood into pulp, which is then dried to makepaper. Black liquor is an aqueous solution of lignin residues,hemicellulose, and inorganic chemicals.

U.S. Pat. No. RE 35,377, also incorporated herein by reference, providesa method for the production of methanol by conversion of carbonaceousmaterials such as oil, coal, natural gas and biomass materials. Theprocess includes hydrogasification of solid and/or liquid carbonaceousmaterials to obtain a process gas which is steam pyrolized withadditional natural gas to form synthesis gas. The syngas is converted tomethanol which may be carbonylated to acetic acid. The method likewiseproduces hydrogen which may be used in connection with this invention asnoted above. U.S. Pat. No. 5,821,111, which discloses a process forconverting waste biomass through gasification into synthesis gas, andU.S. Pat. No. 6,685,754, which discloses a method for the production ofa hydrogen-containing gas composition, such as a synthesis gas includinghydrogen and carbon monoxide, are incorporated herein by reference intheir entireties.

The acetic acid fed to the hydrogenation reactor may also comprise othercarboxylic acids and anhydrides, as well as aldehydes and/or ketones,such as acetaldehyde and acetone. Preferably, a suitable acetic acidfeed stream comprises one or more of the compounds selected from thegroup consisting of acetic acid, acetic anhydride, acetaldehyde, ethylacetate, and mixtures thereof. These other compounds may also behydrogenated in the processes of the present invention. In someembodiments, the presence of carboxylic acids, such as propanoic acid orits anhydride, may be beneficial in producing propanol. Water may alsobe present in the acetic acid feed.

Alternatively, acetic acid in vapor form may be taken directly as crudeproduct from the flash vessel of a methanol carbonylation unit of theclass described in U.S. Pat. No. 6,657,078, the entirety of which isincorporated herein by reference. The crude vapor product, for example,may be fed directly to the hydrogenation reactor without the need forcondensing the acetic acid and light ends or removing water, savingoverall processing costs.

The acetic acid may be vaporized at the reaction temperature, followingwhich the vaporized acetic acid may be fed along with hydrogen in anundiluted state or diluted with a relatively inert carrier gas, such asnitrogen, argon, helium, carbon dioxide and the like. For reactions runin the vapor phase, the temperature should be controlled in the systemsuch that it does not fall below the dew point of acetic acid. In oneembodiment, the acetic acid may be vaporized at the boiling point ofacetic acid at the particular pressure, and then the vaporized aceticacid may be further heated to the reactor inlet temperature. In anotherembodiment, the acetic acid is mixed with other gases before vaporizing,followed by heating the mixed vapors up to the reactor inlettemperature. Preferably, the acetic acid is transferred to the vaporstate by passing hydrogen and/or recycle gas through the acetic acid ata temperature at or below 125° C., followed by heating of the combinedgaseous stream to the reactor inlet temperature.

The reactor, in some embodiments, may include a variety ofconfigurations using a fixed bed reactor or a fluidized bed reactor. Inmany embodiments of the present invention, an “adiabatic” reactor can beused; that is, there is little or no need for internal plumbing throughthe reaction zone to add or remove heat. In other embodiments, a radialflow reactor or reactors may be employed as the reactor, or a series ofreactors may be employed with or without heat exchange, quenching, orintroduction of additional feed material. Alternatively, a shell andtube reactor provided with a heat transfer medium may be used. In manycases, the reaction zone may be housed in a single vessel or in a seriesof vessels with heat exchangers therebetween.

In preferred embodiments, the catalyst is employed in a fixed bedreactor, e.g., in the shape of a pipe or tube, where the reactants,typically in the vapor form, are passed over or through the catalyst.Other reactors, such as fluid or ebullient bed reactors, can beemployed. In some instances, the hydrogenation catalysts may be used inconjunction with an inert material to regulate the pressure drop of thereactant stream through the catalyst bed and the contact time of thereactant compounds with the catalyst particles.

The hydrogenation in the reactor may be carried out in either the liquidphase or vapor phase. Preferably, the reaction is carried out in thevapor phase under the following conditions. The reaction temperature mayrange from 125° C. to 350° C., e.g., from 200° C. to 325° C., from 225°C. to 300° C., or from 250° C. to 300° C. The pressure may range from 10kPa to 3000 kPa, e.g., from 50 kPa to 2300 kPa, or from 100 kPa to 1500kPa. The reactants may be fed to the reactor at a gas hourly spacevelocity (GHSV) of greater than 500 hr⁻¹, e.g., greater than 1000 hr⁻¹,greater than 2500 hr⁻¹ or even greater than 5000 hr⁻¹. In terms ofranges the GHSV may range from 50 hr⁻¹ to 50,000 hr⁻¹, e.g., from 500hr⁻¹ to 30,000 hr⁻¹, from 1000 hr⁻¹ to 10,000 hr⁻¹, or from 1000 hr⁻¹ to6500 hr⁻¹.

The hydrogenation optionally is carried out at a pressure justsufficient to overcome the pressure drop across the catalytic bed at theGHSV selected, although there is no bar to the use of higher pressures,it being understood that considerable pressure drop through the reactorbed may be experienced at high space velocities, e.g., 5000 hr⁻¹ or6,500 hr⁻¹.

Although the reaction consumes two moles of hydrogen per mole of aceticacid to produce one mole of ethanol, the actual molar ratio of hydrogento acetic acid in the feed stream may vary from about 100:1 to 1:100,e.g., from 50:1 to 1:50, from 20:1 to 1:2, or from 12:1 to 1:1. Mostpreferably, the molar ratio of hydrogen to acetic acid is greater than2:1, e.g., greater than 4:1 or greater than 8:1.

Contact or residence time can also vary widely, depending upon suchvariables as amount of acetic acid, catalyst, reactor, temperature, andpressure. Typical contact times range from a fraction of a second tomore than several hours when a catalyst system other than a fixed bed isused, with preferred contact times, at least for vapor phase reactions,of from 0.1 to 100 seconds, e.g., from 0.3 to 80 seconds or from 0.4 to30 seconds.

The hydrogenation of acetic acid to form ethanol is preferably conductedin the presence of a hydrogenation catalyst in the reactor. Suitablehydrogenation catalysts include catalysts comprising a first metal andoptionally one or more of a second metal, a third metal or any number ofadditional metals, optionally on a catalyst support. The first andoptional second and third metals may be selected from Group IB, IIB,IIIB, IVB, VB, VIIB, VIIB, VIII transition metals, a lanthanide metal,an actinide metal or a metal selected from any of Groups IIIA, IVA, VA,and VIA. Preferred metal combinations for some exemplary catalystcompositions include platinum/tin, platinum/ruthenium, platinum/rhenium,palladium/ruthenium, palladium/rhenium, cobalt/palladium,cobalt/platinum, cobalt/chromium, cobalt/ruthenium, cobalt/tin,silver/palladium, copper/palladium, copper/zinc, nickel/palladium,gold/palladium, ruthenium/rhenium, and ruthenium/iron. Exemplarycatalysts are further described in U.S. Pat. No. 7,608,744 and U.S. Pub.No. 2010/0029995, the entireties of which are incorporated herein byreference. In another embodiment, the catalyst comprises a Co/Mo/Scatalyst of the type described in U.S. Pub. No. 2009/0069609, theentirety of which is incorporated herein by reference.

In one embodiment, the catalyst comprises a first metal selected fromthe group consisting of copper, iron, cobalt, nickel, ruthenium,rhodium, palladium, osmium, iridium, platinum, titanium, zinc, chromium,rhenium, molybdenum, and tungsten. Preferably, the first metal isselected from the group consisting of platinum, palladium, cobalt,nickel, and ruthenium. More preferably, the first metal is selected fromplatinum and palladium. In embodiments of the invention where the firstmetal comprises platinum, it is preferred that the catalyst comprisesplatinum in an amount less than 5 wt. %, e.g., less than 3 wt. % or lessthan 1 wt. %, due to the high commercial demand for platinum.

As indicated above, in some embodiments, the catalyst further comprisesa second metal, which typically would function as a promoter. Ifpresent, the second metal preferably is selected from the groupconsisting of copper, molybdenum, tin, chromium, iron, cobalt, vanadium,tungsten, palladium, platinum, lanthanum, cerium, manganese, ruthenium,rhenium, gold, and nickel. More preferably, the second metal is selectedfrom the group consisting of copper, tin, cobalt, rhenium, and nickel.More preferably, the second metal is selected from tin and rhenium.

In certain embodiments where the catalyst includes two or more metals,e.g., a first metal and a second metal, the first metal is present inthe catalyst in an amount from 0.1 to 10 wt. %, e.g., from 0.1 to 5 wt.%, or from 0.1 to 3 wt. %. The second metal preferably is present in anamount from 0.1 to 20 wt. %, e.g., from 0.1 to 10 wt. %, or from 0.1 to5 wt. %. For catalysts comprising two or more metals, the two or moremetals may be alloyed with one another or may comprise a non-alloyedmetal solution or mixture.

The preferred metal ratios may vary depending on the metals used in thecatalyst. In some exemplary embodiments, the mole ratio of the firstmetal to the second metal is from 10:1 to 1:10, e.g., from 4:1 to 1:4,from 2:1 to 1:2, from 1.5:1 to 1:1.5 or from 1.1:1 to 1:1.1.

The catalyst may also comprise a third metal selected from any of themetals listed above in connection with the first or second metal, solong as the third metal is different from the first and second metals.In preferred aspects, the third metal is selected from the groupconsisting of cobalt, palladium, ruthenium, copper, zinc, platinum, tin,and rhenium. More preferably, the third metal is selected from cobalt,palladium, and ruthenium. When present, the total weight of the thirdmetal preferably is from 0.05 to 4 wt. %, e.g., from 0.1 to 3 wt. %, orfrom 0.1 to 2 wt. %.

In addition to one or more metals, in some embodiments of the presentinvention the catalysts further comprise a support or a modifiedsupport. As used herein, the term “modified support” refers to a supportthat includes a support material and a support modifier, which adjuststhe acidity of the support material.

The total weight of the support or modified support, based on the totalweight of the catalyst, preferably is from 75 to 99.9 wt. %, e.g., from78 to 97 wt. %, or from 80 to 95 wt. %. In preferred embodiments thatutilize a modified support, the support modifier is present in an amountfrom 0.1 to 50 wt. %, e.g., from 0.2 to 25 wt. %, from 0.5 to 15 wt. %,or from 1 to 8 wt. %, based on the total weight of the catalyst. Themetals of the catalysts may be dispersed throughout the support, layeredthroughout the support, coated on the outer surface of the support(i.e., egg shell), or decorated on the surface of the support.

As will be appreciated by those of ordinary skill in the art, supportmaterials are selected such that the catalyst system is suitably active,selective and robust under the process conditions employed for theformation of ethanol.

Suitable support materials may include, for example, stable metaloxide-based supports or ceramic-based supports. Preferred supportsinclude silicaceous supports, such as silica, silica/alumina, a GroupIIA silicate such as calcium metasilicate, pyrogenic silica, high puritysilica, and mixtures thereof. Other supports may include, but are notlimited to, iron oxide, alumina, titania, zirconia, magnesium oxide,carbon, graphite, high surface area graphitized carbon, activatedcarbons, and mixtures thereof.

As indicated, the catalyst support may be modified with a supportmodifier. In some embodiments, the support modifier may be an acidicmodifier that increases the acidity of the catalyst. Suitable acidicsupport modifiers may be selected from the group consisting of: oxidesof Group IVB metals, oxides of Group VB metals, oxides of Group VIBmetals, oxides of Group VIIB metals, oxides of Group VIIIB metals,aluminum oxides, and mixtures thereof. Acidic support modifiers includethose selected from the group consisting of TiO₂, ZrO₂, Nb₂O₅, Ta₂O₅,Al₂O₃, B₂O₃, P₂O₅, and Sb₂O₃. Preferred acidic support modifiers includethose selected from the group consisting of TiO₂, ZrO₂, Nb₂O₅, Ta₂O₅,and Al₂O₃. The acidic modifier may also include those selected from thegroup consisting of WO₃, MoO₃, Fe₂O₃, Cr₂O₃, V₂O₅, MnO₂, CuO, Co₂O₃, andBi₂O₃.

In another embodiment, the support modifier may be a basic modifier thathas a low volatility or no volatility. Such basic modifiers, forexample, may be selected from the group consisting of: (i) alkalineearth metal oxides, (ii) alkali metal oxides, (iii) alkaline earth metalmetasilicates, (iv) alkali metal metasilicates, (v) Group IIB metaloxides, (vi) Group IIB metal metasilicates, (vii) Group IIIB metaloxides, (viii) Group IIIB metal metasilicates, and mixtures thereof. Inaddition to oxides and metasilicates, other types of modifiers includingnitrates, nitrites, acetates, and lactates may be used. Preferably, thesupport modifier is selected from the group consisting of oxides andmetasilicates of any of sodium, potassium, magnesium, calcium, scandium,yttrium, and zinc, as well as mixtures of any of the foregoing. Morepreferably, the basic support modifier is a calcium silicate, and evenmore preferably calcium metasilicate (CaSiO₃). If the basic supportmodifier comprises calcium metasilicate, it is preferred that at least aportion of the calcium metasilicate is in crystalline form.

A preferred silica support material is SS61138 High Surface Area (HSA)Silica Catalyst Carrier from Saint-Gobain NorPro. The Saint-GobainNorPro SS61138 silica exhibits the following properties: containsapproximately 95 wt. % high surface area silica; surface area of about250 m²/g; median pore diameter of about 12 nm; average pore volume ofabout 1.0 cm³/g as measured by mercury intrusion porosimetry and apacking density of about 0.352 g/cm³ (22 lb/ft³).

A preferred silica/alumina support material is KA-160 silica spheresfrom Sud Chemie having a nominal diameter of about 5 mm, a density ofabout 0.562 g/ml, an absorptivity of about 0.583 g H₂O/g support, asurface area of about 160 to 175 m²/g, and a pore volume of about 0.68ml/g.

The catalyst compositions suitable for use with the present inventionpreferably are formed through metal impregnation of the modifiedsupport, although other processes such as chemical vapor deposition mayalso be employed. Such impregnation techniques are described in U.S.Pat. Nos. 7,608,744 and 7,863,489 and U.S. Pub. No. 2010/0197485referred to above, the entireties of which are incorporated herein byreference.

In particular, the hydrogenation of acetic acid may achieve favorableconversion of acetic acid and favorable selectivity and productivity toethanol in the reactor. For purposes of the present invention, the term“conversion” refers to the amount of acetic acid in the feed that isconverted to a compound other than acetic acid. Conversion is expressedas a mole percentage based on acetic acid in the feed. As indicatedabove, the conversion for the first embodiment is from 40% to 70%, andthe conversion for the second embodiment is greater than 85%.

Selectivity is expressed as a mole percent based on converted aceticacid. It should be understood that each compound converted from aceticacid has an independent selectivity and that selectivity is independentfrom conversion. For example, if 60 mole % of the converted acetic acidis converted to ethanol, we refer to the ethanol selectivity as 60%.Preferably, the catalyst selectivity to ethoxylates is at least 60%,e.g., at least 70%, or at least 80%. As used herein, the term“ethoxylates” refers specifically to the compounds ethanol,acetaldehyde, and ethyl acetate. Preferably, in the reactor, theselectivity to ethanol is at least 80%, e.g., at least 85% or at least88%. Preferred embodiments of the hydrogenation process also have lowselectivity to undesirable products, such as methane, ethane, and carbondioxide. The selectivity to these undesirable products preferably isless than 4%, e.g., less than 2% or less than 1%. More preferably, theseundesirable products are present in undetectable amounts. Formation ofalkanes may be low, and ideally less than 2%, less than 1%, or less than0.5% of the acetic acid passed over the catalyst is converted toalkanes, which have little value other than as fuel.

The term “productivity,” as used herein, refers to the grams of aspecified product, e.g., ethanol, formed during the hydrogenation basedon the kilograms of catalyst used per hour. A productivity of at least100 grams of ethanol per kilogram of catalyst per hour, e.g., at least400 grams of ethanol per kilogram of catalyst per hour or at least 600grams of ethanol per kilogram of catalyst per hour, is preferred. Interms of ranges, the productivity preferably is from 100 to 3,000 gramsof ethanol per kilogram of catalyst per hour, e.g., from 400 to 2,500grams of ethanol per kilogram of catalyst per hour or from 600 to 2,000grams of ethanol per kilogram of catalyst per hour.

Operating under the conditions of the present invention may result inethanol production on the order of at least 0.1 tons of ethanol perhour, e.g., at least 1 ton of ethanol per hour, at least 5 tons ofethanol per hour, or at least 10 tons of ethanol per hour. Larger scaleindustrial production of ethanol, depending on the scale, generallyshould be at least 1 ton of ethanol per hour, e.g., at least 15 tons ofethanol per hour or at least 30 tons of ethanol per hour. In terms ofranges, for large scale industrial production of ethanol, the process ofthe present invention may produce from 0.1 to 160 tons of ethanol perhour, e.g., from 15 to 160 tons of ethanol per hour or from 30 to 80tons of ethanol per hour. Ethanol production from fermentation, due theeconomies of scale, typically does not permit the single facilityethanol production that may be achievable by employing embodiments ofthe present invention.

In various embodiments of the present invention, the crude ethanolmixture produced by the reactor, before any subsequent processing, suchas purification and separation, will typically comprise acetic acid,ethanol and water. As used herein, the term “crude ethanol mixture”refers to any composition comprising from 5 to 70 wt. % ethanol and from5 to 40 wt. % water. Exemplary compositional ranges for the crudeethanol mixture are provided in Table 1. The “others” identified inTable 1 may include, for example, esters, ethers, aldehydes, ketones,alkanes, and carbon dioxide.

TABLE 1 CRUDE ETHANOL MIXTURE COMPOSITIONS Conc. Conc. Conc. Conc.Component (wt. %) (wt. %) (wt. %) (wt. %) Ethanol 5 to 70 15 to 70   15to 50 25 to 50 Acetic Acid 0 to 90 0 to 50   5 to 70  5 to 50 Water 5 to30 5 to 28  10 to 26 10 to 22 Ethyl Acetate 0 to 30 0 to 20   1 to 12  3to 10 Acetaldehyde 0 to 10 0 to 3  0.1 to 3 0.2 to 2  Diethyl Acetal0.001 to 5    0.01 to 3    0.1 to 2 0.5 to 1.5 Others 0.1 to 10  0.1 to6   0.1 to 4 —

In one embodiment, the crude ethanol mixture may comprise acetic acid inan amount less than 20 wt. %, e.g., less than 15 wt. %, less than 10 wt.% or less than 5 wt. %. In embodiments having lower amounts of aceticacid, the conversion of acetic acid is preferably greater than 75%,e.g., greater than 85% or greater than 90%. In addition, the selectivityto ethanol may also be preferably high, and is greater than 75%, e.g.,greater than 85% or greater than 90%.

Ethanol Separation

Ethanol produced may be recovered using several different techniques. InFIG. 1, the separation of the crude ethanol mixture uses four columns.In one embodiment, the first column 120, second column 123, and/or thirdcolumn 128 may be operated at an increased pressure. In one embodiment,second column 123 may have a stripping section comprising at least 40stages, e.g., at least 50 stages or at least 60 stages.

In FIG. 2, the crude ethanol mixture is separated in two columns with anintervening water separation. Either column in FIG. 2 may be operated anincreased pressure. FIG. 3 is similar separation as FIG. 2, exceptethanol containing stream is separated in parallel using a columnoperating at vacuum conditions and a column operating at an increasedpressure. FIG. 4 separates an ethanol containing stream in series usinga low pressure column, followed by a column operating at an increasedpressure. FIG. 5 separates an ethanol containing stream in series usinga column operating at an increased pressure followed by separating thedistillate in a low pressure column. In FIGS. 4 and 5, the residue ofthe low pressure column may be combined with the residue of the columnoperated at an increased pressure to form an ethanol product.

In FIG. 6, the separation of the crude ethanol mixture uses two columns.Either column in FIG. 6 may be operated an increased pressure, but itmay be preferred to operate the first column 170 at a higher pressure.

Other separation systems may also be used with embodiments of thepresent invention. For purposes of convenience, the columns in eachexemplary separation processes, may be referred as the first column,second columns, third columns, etc.

In each of the FIGS., hydrogenation system 100 includes a reaction zone101, and separation zone 102. Hydrogen and acetic acid via lines 104 and105, respectively, are fed to a vaporizer 106 to create a vapor feedstream in line 107 that is directed to reactor 103. In one embodiment,lines 104 and 105 may be combined and jointly fed to the vaporizer 106.The temperature of the vapor feed stream in line 107 is preferably from100° C. to 350° C., e.g., from 120° C. to 310° C. or from 150° C. to300° C. Any feed that is not vaporized is removed from vaporizer 106 andmay be recycled or discarded thereto. In addition, although line 107 isshown as being directed to the top of reactor 103, line 107 may bedirected to the side, upper portion, or bottom of reactor 103.

Reactor 103 contains the catalyst that is used in the hydrogenation ofthe carboxylic acid, preferably acetic acid. In one embodiment, one ormore guard beds (not shown) may be used upstream of the reactor,optionally upstream of the vaporizer 106, to protect the catalyst frompoisons or undesirable impurities contained in the feed orreturn/recycle streams. Such guard beds may be employed in the vapor orliquid streams. Suitable guard bed materials may include, for example,carbon, silica, alumina, ceramic, or resins. In one aspect, the guardbed media is functionalized, e.g., silver functionalized, to trapparticular species such as sulfur or halogens. During the hydrogenationprocess, a crude ethanol mixture stream is withdrawn, preferablycontinuously, from reactor 103 via line 109.

The crude ethanol mixture stream in line 109 may be condensed and fed toa separator 110, which, in turn, provides a vapor stream 111 and aliquid stream 112. In some embodiments, separator 110 may comprise aflasher or a knockout pot. The separator 110 may operate at atemperature of from 20° C. to 250° C., e.g., from 30° C. to 225° C. orfrom 60° C. to 200° C. The pressure of separator 110 may be from 50 kPato 2000 kPa, e.g., from 75 kPa to 1500 kPa or from 100 kPa to 1000 kPa.Optionally, the crude ethanol mixture in line 109 may pass through oneor more membranes to separate hydrogen and/or other non-condensablegases.

The vapor stream 111 exiting separator 110 may comprise hydrogen andhydrocarbons, and may be purged and/or returned to reaction zone 101.When returned to reaction zone 101, vapor stream 110 is combined withthe hydrogen feed 104 and co-fed to vaporizer 106. In some embodiments,the returned vapor stream 111 may be compressed before being combinedwith hydrogen feed 104.

In FIG. 1, the liquid stream 112 from separator 110 is withdrawn andpumped to the side of first column 120, also referred to as an “acidseparation column.” In one embodiment, the contents of liquid stream 112are substantially similar to the crude ethanol mixture obtained from thereactor, except that the composition has been depleted of hydrogen,carbon dioxide, methane and/or ethane, which are removed by separator110. Accordingly, liquid stream 112 may also be referred to as a crudeethanol mixture. Exemplary components of liquid stream 112 are providedin Table 2. It should be understood that liquid stream 112 may containother components, not listed in Table 2.

TABLE 2 COLUMN FEED COMPOSITION (Liquid Stream 112) Conc. (wt. %) Conc.(wt. %) Conc. (wt. %) Ethanol 5 to 70    10 to 60 15 to 50 Acetic Acid<90     5 to 80  5 to 70 Water 5 to 30    5 to 28 10 to 26 Ethyl Acetate<30   0.001 to 20  1 to 12 Acetaldehyde <10  0.001 to 3 0.1 to 3  Acetal<5 0.001 to 2 0.005 to 1    Acetone <5  0.0005 to 0.05 0.001 to 0.03 Other Esters <5 <0.005 <0.001 Other Ethers <5 <0.005 <0.001 OtherAlcohols <5 <0.005 <0.001

The amounts indicated as less than (<) in the tables throughout presentspecification are preferably not present and if present may be presentin trace amounts or in amounts greater than 0.0001 wt. %.

The “other esters” in Table 2 may include, but are not limited to, ethylpropionate, methyl acetate, isopropyl acetate, n-propyl acetate, n-butylacetate or mixtures thereof. The “other ethers” in Table 2 may include,but are not limited to, diethyl ether, methyl ethyl ether, isobutylethyl ether or mixtures thereof. The “other alcohols” in Table 2 mayinclude, but are not limited to, methanol, isopropanol, n-propanol,n-butanol or mixtures thereof. In one embodiment, the liquid stream 112may comprise propanol, e.g., isopropanol and/or n-propanol, in an amountfrom 0.001 to 0.1 wt. %, from 0.001 to 0.05 wt. % or from 0.001 to 0.03wt. %. In should be understood that these other components may becarried through in any of the distillate or residue streams describedherein and will not be further described herein, unless indicatedotherwise.

Optionally, crude ethanol mixture in line 109 or in liquid stream 112may be further fed to an esterification reactor, hydrogenolysis reactor,or combination thereof. An esterification reactor may be used to consumeresidual acetic acid present in the crude ethanol mixture to furtherreduce the amount of acetic acid that would otherwise need to beremoved. Hydrogenolysis may be used to convert ethyl acetate in thecrude ethanol mixture to ethanol.

In the embodiment shown in FIG. 1, line 112 is introduced in the lowerpart of first column 120, e.g., lower half or lower third. In firstcolumn 120, acetic acid, a portion of the water, and other heavycomponents, if present, are removed from the composition in line 121 andare withdrawn, preferably continuously, as residue. Some or all of theresidue may be returned and/or recycled back to reaction zone 101 vialine 121. Recycling the acetic acid in line 121 to the vaporizer 106 mayreduce the amount of heavies that need to be purged from vaporizer 106.Reducing the amount of heavies to be purged may improve efficiencies ofthe process while reducing byproducts.

First column 120 also forms an overhead distillate, which is withdrawnin line 122, and which may be condensed and refluxed, for example, at aratio of from 10:1 to 1:10, e.g., from 3:1 to 1:3 or from 1:2 to 2:1.The distillate in line 122 comprises primarily ethanol, as well aswater, ethyl acetate, acetaldehyde, and/or diethyl acetal. For example,distillate may comprise from 20 to 75 wt. % ethanol and 10 to 40 wt. %ethanol. Preferably, the concentration of acetic acid in the distillateis less than 2 wt. %, e.g., less than 1 wt. % or less than 0.5 wt. %.

In one embodiment, column 120 may operate at an increased pressure ofabove atmospheric pressure to enhance the hydrolysis of acetal fed tocolumn 120. When column 120 is operated at increased pressure, theconcentration of diethyl acetal in the first distillate in line 121 maybe less than 1 wt. %, e.g., less than 0.1 wt. % or less than 0.01 wt. %.

When another column the purification section 102 of FIG. 1, is operatedat an increased pressure, first column 120 may be operated at ambientpressure. In other embodiments, the pressure of first column 120 mayrange from 0.1 kPa to 510 kPa, e.g., from 1 kPa to 475 kPa or from 1 kPato 375 kPa. When first column 120 is operated under standard atmosphericpressure, the temperature of the residue exiting in line 121 preferablyis from 95° C. to 120° C., e.g., from 110° C. to 117° C. or from 111° C.to 115° C. The temperature of the distillate exiting in line 122preferably is from 70° C. to 110° C., e.g., from 75° C. to 95° C. orfrom 80° C. to 90° C.

In one embodiment, even without operating first column 120 underincreased pressure, without being bound by theory, it has surprisinglyand unexpectedly been discovered that when any amount of acetal isdetected in the feed, the acetal appears to decompose in the column suchthat less or even no detectable amounts are present in the distillateand/or residue. Increasing the pressure of first column 120 may furtherreduce the concentrations of acetal. In one embodiment, from 10 to 75%of the diethyl acetal may be decomposed in first column 120, e.g., from15 to 60% or more preferably from 20 to 40%.

To further separate distillate, line 122 is introduced to the secondcolumn 123, also referred to as the “light ends column,” preferably inthe middle part of column 123. Preferably the second column 123 is anextractive distillation column, and an extraction agent is added theretovia lines 124 and/or 125. Extractive distillation is a method ofseparating close boiling components, such as azeotropes, by distillingthe feed in the presence of an extraction agent. The extraction agentpreferably has a boiling point that is higher than the compounds beingseparated in the feed. In preferred embodiments, the extraction agent iscomprised primarily of water. As indicated above, the first distillatein line 122 that is fed to the second column 123 comprises ethanol,water, and ethyl acetate. These compounds tend to form binary andternary azeotropes, which decrease separation efficiency. As shown, inone embodiment the extraction agent comprises the third residue in line124. Preferably, the recycled third residue in line 124 is fed to secondcolumn 123 at a point higher than the first distillate in line 122. Inone embodiment, the recycled third residue in line 124 is fed near thetop of second column 123 or fed, for example, above the feed in line 122and below the reflux line from the condensed overheads. In a traycolumn, the third residue in line 124 is continuously added near the topof the second column 123 so that an appreciable amount of the thirdresidue is present in the liquid phase on all of the trays below. Inanother embodiment, the extraction agent is fed from a source outside ofthe process 100 via line 125 to second column 123. Preferably thisextraction agent comprises water.

The molar ratio of the water in the extraction agent to the ethanol inthe feed to the second column is preferably at least 0.5:1, e.g., atleast 1:1 or at least 3:1. In terms of ranges, preferred molar ratiosmay range from 0.5:1 to 8:1, e.g., from 1:1 to 7:1 or from 2:1 to 6.5:1.Higher molar ratios may be used but with diminishing returns in terms ofthe additional ethyl acetate in the second distillate and decreasedethanol concentrations in the second column distillate.

In one embodiment, an additional extraction agent, such as water from anexternal source, dimethylsulfoxide, glycerine, diethylene glycol,1-naphthol, hydroquinone, N,N′-dimethylformamide, 1,4-butanediol;ethylene glycol-1,5-pentanediol; propylene glycol-tetraethyleneglycol-polyethylene glycol; glycerine-propylene glycol-tetraethyleneglycol-1,4-butanediol, ethyl ether, methyl formate, cyclohexane,N,N′-dimethyl-1,3-propanediamine, N,N′-dimethylethylenediamine,diethylene triamine, hexamethylene diamine and 1,3-diaminopentane, analkylated thiopene, dodecane, tridecane, tetradecane and chlorinatedparaffins, may be added to second column 123. Some suitable extractionagents include those described in U.S. Pat. Nos. 4,379,028, 4,569,726,5,993,610 and 6,375,807, the entire contents and disclosure of which arehereby incorporated by reference. The additional extraction agent may becombined with the recycled third residue in line 124 and co-fed to thesecond column 123. The additional extraction agent may also be addedseparately to the second column 123. In one aspect, the extraction agentcomprises an extraction agent, e.g., water, derived from an externalsource via line 125 and none of the extraction agent is derived from thethird residue.

Second column 123 may be a tray or packed column. In one embodiment,second column 123 is a tray column having from 5 to 120 trays, e.g.,from 15 to 80 trays or from 20 to 70 trays. In one embodiment, thestripping section of the second column 123 may have at least 40 stages,e.g. at least 50 stages or at least 60 stages. As indicated above, theincreased stripping section of second column 123 may enhance hydrolysisof diethyl acetal.

In FIG. 1, second column 123 may operate at an increased pressure, i.e.greater than atmospheric pressure. In other embodiments, when either thefirst column 123 or third column 128 is operated at increased pressure,the pressure of second column 123 may range from 0.1 kPa to 510 kPa,e.g., from 1 kPa to 475 kPa or from 1 kPa to 375 kPa.

The temperature of second column 123 at atmospheric pressure may vary.In one embodiment second residue exiting in line 126 preferably is at atemperature from 60° C. to 90° C., e.g., from 70° C. to 90° C. or from80° C. to 90° C. The temperature of the second distillate exiting inline 127 from second column 123 preferably is from 50° C. to 90° C.,e.g., from 60° C. to 80° C. or from 60° C. to 70° C.

The second residue in line 126 comprises ethanol and water. The secondresidue may comprise less than 3 wt. % ethyl acetate, e.g., less than 1wt. % ethyl acetate or less than 0.5 wt. % ethyl acetate. The seconddistillate in line 127 comprises ethyl acetate, acetaldehyde, and/ordiethyl acetal. In addition, minor amounts of ethanol may be present inthe second distillate. The weight ratio of ethanol in the second residueto second distillate preferably is at least 3:1, e.g., at least 6:1, atleast 8:1, at least 10:1 or at least 15:1.

All or a portion of the third residue is recycled to the second column.In one embodiment, all of the third residue may be recycled untilprocess 100 reaches a steady state and then a portion of the thirdresidue is recycled with the remaining portion being purged from thesystem 100. The composition of the second residue will tend to havelower amounts of ethanol than when the third residue is not recycled. Asthe third residue is recycled, the composition of the second residuecomprises less than 30 wt. % of ethanol, e.g., less than 20 wt. % orless than 15 wt. %. The majority of the second residue preferablycomprises water. Notwithstanding this effect, the extractivedistillation step advantageously also reduces the amount of ethylacetate that is sent to the third column, which is highly beneficial inultimately forming a highly pure ethanol product.

As shown, the second residue from second column 123, which comprisesethanol and water, is fed via line 126 to third column 128, alsoreferred to as the “product column.” More preferably, the second residuein line 126 is introduced in the lower part of third column 128, e.g.,lower half or lower third. Third column 128 recovers ethanol, whichpreferably is substantially pure with respect to organic impurities andother than the azeotropic water content, as the distillate in line 129.The distillate of third column 128 preferably is refluxed as shown inFIG. 1, for example, at a reflux ratio of from 1:10 to 10:1, e.g., from1:3 to 3:1 or from 1:2 to 2:1. The third residue in line 124, whichcomprises primarily water, preferably is returned to the second column123 as an extraction agent as described above. In one embodiment, afirst portion of the third residue in line 124 is recycled to the secondcolumn and a second portion is purged and removed from the system vialine 130. In one embodiment, once the process reaches steady state, thesecond portion of water to be purged is substantially similar to theamount of water formed in the hydrogenation of acetic acid. In oneembodiment, a portion of the third residue may be used to hydrolyze anyother stream, such as one or more streams comprising ethyl acetate. Inone embodiment, the third residue in line 124 is withdrawn from thirdcolumn 128 at a temperature higher than the operating temperature of thesecond column 123. Preferably, the third residue in line 124 isintegrated to heat one or more other streams or is reboiled prior to bereturned to the second column 123.

Although FIG. 1 show third residue being directly recycled to secondcolumn 123, third residue may also be returned indirectly, for example,by storing a portion or all of the third residue in a tank (not shown)or treating the third residue to further separate any minor componentssuch as aldehydes, higher molecular weight alcohols, or esters in one ormore additional columns (not shown).

Third column 128 is preferably a tray column. In one embodiment, thirdcolumn 128 may operate at an increased pressure to reduce diethyl acetalconcentration. When either first column 120 or second column 123 isoperated at increased pressure, third column may be operated from 0.1kPa to 510 kPa, e.g., from 1 kPa to 475 kPa or from 1 kPa to 375 kPa.

At atmospheric pressure, the temperature of the third distillate exitingin line 129 preferably is from 60° C. to 110° C., e.g., from 70° C. to100° C. or from 75° C. to 95° C. The temperature of the third residue inline 124 preferably is from 70° C. to 115° C., e.g., from 80° C. to 110°C. or from 85° C. to 105° C.

Any of the compounds that are carried through the distillation processfrom the feed or crude reaction product generally remain in the thirddistillate in amounts of less 0.1 wt. %, based on the total weight ofthe third distillate composition, e.g., less than 0.05 wt. % or lessthan 0.02 wt. %. In one embodiment, one or more side streams may removeimpurities from any of the columns in the system 100. Preferably atleast one side stream is used to remove impurities from the third column128. The impurities may be purged and/or retained within the system 100.The ethanol product obtained from third distillate in FIG. 1, is shownbelow in Table 3. Preferably, the ethanol product comprises less than 1wt. % diethyl acetal, e.g., less than 0.5 wt. % or less than 0.01 wt. %.

The third distillate in line 129 may be further purified to form ananhydrous ethanol product stream, i.e., “finished anhydrous ethanol,”using one or more additional separation systems, such as, for example,distillation columns, adsorption units, membranes, or molecular sieves.Suitable adsorption units include pressure swing adsorption units andthermal swing adsorption unit.

In FIG. 1, any of first column 120, second column 123, or third column128 may be operated at increased pressure, e.g., from 101 kPa to 5,000kPa, e.g., from 120 kPa to 4,000 kPa, or from 150 kPa to 3,000 kPa.Preferably, at least second column 123 is operated at increasedpressure. In some embodiments two of the columns may be operated atincreased pressure, such as the second column 123 and third column 128.

Returning to second column 123, the second distillate preferably isrefluxed as shown in FIG. 1, at a reflux ratio of 1:10 to 10:1, e.g.,from 1:5 to 5:1 or from 1:3 to 3:1. The second distillate in line 127may be purged or recycled to the reaction zone. The second distillate inline 127 may be further processed in an optional fourth column 131, alsoreferred to as the “acetaldehyde removal column.” In optional fourthcolumn 131 the second distillate is separated into a fourth distillate,which comprises acetaldehyde, in line 132 and a fourth residue, whichcomprises ethyl acetate, in line 133. The fourth distillate preferablyis refluxed at a reflux ratio of from 1:20 to 20:1, e.g., from 1:15 to15:1 or from 1:10 to 10:1, and a portion of the fourth distillate isreturned to the reaction zone 101. For example, the fourth distillatemay be combined with the acetic acid feed, added to the vaporizer 106,or added directly to the reactor 103. The fourth distillate preferablyis co-fed with the acetic acid in feed line 105 to vaporizer 106.Without being bound by theory, since acetaldehyde may be hydrogenated toform ethanol, the recycling of a stream that contains acetaldehyde tothe reaction zone increases the yield of ethanol and decreases byproductand waste generation. In another embodiment, the acetaldehyde may becollected and utilized, with or without further purification, to makeuseful products including but not limited to n-butanol, 1,3-butanediol,and/or crotonaldehyde and derivatives.

The fourth residue of optional fourth column 131 may be purged via line133. The fourth residue primarily comprises ethyl acetate and ethanol,which may be suitable for use as a solvent mixture or in the productionof esters. In one preferred embodiment, the acetaldehyde is removed fromthe second distillate in fourth column 131 such that no detectableamount of acetaldehyde is present in the residue of column 131.

Optional fourth column 131 is preferably a tray column as describedabove and preferably operates above atmospheric pressure. In oneembodiment, the pressure is from 120 kPa to 5,000 kPa, e.g., from 200kPa to 4,500 kPa, or from 400 kPa to 3,000 kPa. In a preferredembodiment the fourth column 131 may operate at a pressure that ishigher than the pressure of the other columns. Although acetalhydrolysis may be enhanced at increased pressure in optional fourthcolumn 131, because no ethanol is recovered from optional fourth column131 the increased pressure may have little effect on the diethyl acetalconcentration in the ethanol product.

The temperature of the fourth distillate exiting in line 132 preferablyis from 60° C. to 110° C., e.g., from 70° C. to 100° C. or from 75° C.to 95° C. The temperature of the residue in line 133 preferably is from70° C. to 115° C., e.g., from 80° C. to 110° C. or from 85° C. to 110°C.

In one embodiment, a portion of the third residue in line 124 isrecycled to second column 123. In one embodiment, recycling the thirdresidue further reduces the aldehyde components in the second residueand concentrates these aldehyde components in second distillate in line127 and thereby sent to the fourth column 131, wherein the aldehydes maybe more easily separated. The third distillate, e.g. intermediatestream, in line 129 may have lower concentrations of aldehydes andesters due to the recycling of third residue in line 124.

FIG. 2 illustrates another exemplary separation system that has asimilar reaction zone 101 as FIG. 1 and produces a liquid stream 112,e.g., crude ethanol mixture, for further separation. In one preferredembodiment, the reaction zone 101 of FIG. 2 operates at above 70% aceticacid conversion, e.g., above 85% conversion or above 90% conversion.Thus, the acetic acid concentration in the liquid stream 112 may be low.

Liquid stream 112 is fed to the first column 134 to yield a firstdistillate 135 and first residue 136. Liquid stream 112 may beintroduced in the middle or lower portion of first column 134, alsoreferred to as acid-water column. In one embodiment, no entrainers areadded to first column 134. Water and acetic acid, along with any otherheavy components, if present, are removed from liquid stream 112 and arewithdrawn, preferably continuously, as a first residue in line 136.Preferably, a substantial portion of the water in the crude ethanolmixture that is fed to first column 134 may be removed in the firstresidue, for example, up to about 75% or to about 90% of the water fromthe crude ethanol mixture. In one embodiment, 30 to 90% of the water inthe crude ethanol mixture is removed in the residue, e.g., from 40 to88% of the water or from 50 to 84% of the water.

First column 134 may be operated under increased pressure to enhancediethyl acetal separation. When column 134 is operated under about 170kPa, the temperature of the residue exiting in line 136 preferably isfrom 90° C. to 130° C., e.g., from 95° C. to 120° C. or from 100° C. to115° C. The temperature of the distillate exiting in line 135 preferablyis from 60° C. to 90° C., e.g., from 65° C. to 85° C. or from 70° C. to80° C. In some embodiments, the pressure of first column 134 may alsorange from 0.1 kPa to 510 kPa, e.g., from 1 kPa to 475 kPa or from 1 kPato 375 kPa.

The first distillate in line 135 comprises water, in addition to ethanoland other organics. In terms of ranges, the water concentration in thefirst distillate in line 135 preferably is from 4 wt. % to 38 wt. %,e.g., from 7 wt. % to 32 wt. %, or from 7 to 25 wt. %. A portion offirst distillate in line 137 may be condensed and refluxed, for example,at a ratio of from 10:1 to 1:10, e.g., from 3:1 to 1:3 or from 1:2 to2:1. It is understood that reflux ratios may vary with the number ofstages, feed locations, column efficiency and/or feed composition.Operating with a reflux ratio of greater than 3:1 may be less preferredbecause more energy may be required to operate the first column 134. Thecondensed portion of the first distillate may also be fed to a secondcolumn 138.

The remaining portion of the first distillate in line 139 is fed to awater separation unit 140. Water separation unit 140 may be anadsorption unit, membrane, molecular sieves, extractive columndistillation, or a combination thereof. A membrane or an array ofmembranes may also be employed to separate water from the distillate.The membrane or array of membranes may be selected from any suitablemembrane that is capable of removing a permeate water stream from astream that also comprises ethanol and ethyl acetate.

In a preferred embodiment, water separator 140 is a pressure swingadsorption (PSA) unit. The PSA unit is optionally operated at atemperature from 30° C. to 160° C., e.g., from 80° C. to 140° C., and apressure of from 0.01 kPa to 550 kPa, e.g., from 1 kPa to 150 kPa. ThePSA unit may comprise two to five beds. Water separator 140 may removeat least 95% of the water from the portion of first distillate in line139, and more preferably from 99% to 99.99% of the water from the firstdistillate, in a water stream 141. All or a portion of water stream 141may be returned to first column 134 in line 142, where the waterpreferably is ultimately recovered from column 134 in the first residuein line 136. Additionally or alternatively, all or a portion of waterstream 141 may be purged via line 143. The remaining portion of firstdistillate exits the water separator 140 as ethanol mixture stream 144.Ethanol mixture stream 144 may have a low water concentration of lessthan 10 wt. %, e.g., less than 6 wt. % or less than 2 wt. %.

Preferably, ethanol mixture stream 144 is not returned or refluxed tofirst column 135. The condensed portion of the first distillate in line137 may be combined with ethanol mixture stream 144 to control the waterconcentration fed to the second column 138. For example, in someembodiments the first distillate may be split into equal portions, whilein other embodiments, all of the first distillate may be condensed orall of the first distillate may be processed in the water separationunit. In FIG. 2, the condensed portion in line 137 and ethanol mixturestream 144 are co-fed to second column 138. In other embodiments, thecondensed portion in line 137 and ethanol mixture stream 144 may beseparately fed to second column 138. The combined distillate and ethanolmixture has a total water concentration of greater than 0.5 wt. %, e.g.,greater than 2 wt. % or greater than 5 wt. %. In terms of ranges, thetotal water concentration of the combined distillate and ethanol mixturemay be from 0.5 to 15 wt. %, e.g., from 2 to 12 wt. %, or from 5 to 10wt. %.

The second column 138 in FIG. 2, also referred to as the “light endscolumn,” removes ethyl acetate and acetaldehyde from the firstdistillate in line 137 and/or ethanol mixture stream 144. Ethyl acetateand acetaldehyde are removed as a second distillate in line 145 andethanol is removed as the second residue in line 146. Preferably ethanolis recovered with low amounts of ethyl acetate, acetaldehyde, and/oracetal, e.g., less than 1 wt. % or more preferably less than 0.5 wt. %.The ethanol product obtained from second residue in FIG. 2, is shownbelow in Table 3. Preferably, the ethanol product comprises less than 1wt. % diethyl acetal, e.g., less than 0.5 wt. % or less than 0.01 wt. %.

Second column 138 may be a tray column or packed column. In oneembodiment, second column 138 is a tray column having from 5 to 120trays, e.g., from 15 to 100 trays or from 20 to 90 trays. In oneembodiment, there may be at least 40 stages in the stripping section ofsecond column 138, e.g., at least 50 stages or at least 60 stages. Theadditional stages in stripping section may enhance acetal hydrolysis.

In one embodiment, second column 138 operates at an increased pressurefrom 101 kPa to 5,000 kPa, e.g., from 120 kPa to 4,000 kPa, or from 150kPa to 3,000 kPa. The increased pressure in second column 138 mayfurther enhance acetal, in particular diethyl acetal, hydrolysis insecond column 138. When first column 134 operates at increased pressure,the second column 138 may operate at a pressure ranging from 0.1 kPa to510 kPa, e.g., from 10 kPa to 450 kPa or from 50 kPa to 350 kPa.Although the temperature of second column 138 may vary, when at about 20kPa to 70 kPa, the temperature of the second residue exiting in line 146preferably is from 30° C. to 75° C., e.g., from 35° C. to 70° C. or from40° C. to 65° C. The temperature of the second distillate exiting inline 145 preferably is from 20° C. to 55° C., e.g., from 25° C. to 50°C. or from 30° C. to 45° C.

The total concentration of water fed to second column 138 preferably isless than 10 wt. %, as discussed above. When first distillate in line137 and/or ethanol mixture stream 144 comprises minor amounts of water,e.g., less than 1 wt. % or less than 0.5 wt. %, additional water may befed to the second column 138 as an extractive agent in the upper portionof the column. A sufficient amount of water is preferably added via theextractive agent such that the total concentration of water fed tosecond column 138 is from 1 to 10 wt. % water, e.g., from 2 to 6 wt. %,based on the total weight of all components fed to second column 138. Ifthe extractive agent comprises water, the water may be obtained from anexternal source or from an internal return/recycle line from one or moreof the other columns or water separators.

Suitable extractive agents may also include, for example,dimethylsulfoxide, glycerine, diethylene glycol, 1-naphthol,hydroquinone, N,N′-dimethylformamide, 1,4-butanediol; ethyleneglycol-1,5-pentanediol; propylene glycol-tetraethyleneglycol-polyethylene glycol; glycerine-propylene glycol-tetraethyleneglycol-1,4-butanediol, ethyl ether, methyl formate, cyclohexane,N,N-dimethyl-1,3-propanediamine, N,N′-dimethylethylenediamine,diethylene triamine, hexamethylene diamine and 1,3-diaminopentane, analkylated thiopene, dodecane, tridecane, tetradecane, chlorinatedparaffins, or a combination thereof. When extractive agents are used, asuitable recovery system, such as a further distillation column, may beused to recycle the extractive agent.

The second distillate in line 145, which comprises ethyl acetate and/oracetaldehyde, preferably is refluxed as shown in FIG. 2, for example, ata reflux ratio of from 1:30 to 30:1, e.g., from 1:10 to 10:1 or from 1:3to 3:1. In one aspect, not shown, the second distillate 145 or a portionthereof may be returned to reactor 103.

In one embodiment, the second distillate in line 145 and/or a refinedsecond distillate, or a portion of either or both streams, may befurther separated to produce an acetaldehyde-containing stream and anethyl acetate-containing stream. For example, the optional fourth column131 of FIG. 1 may be used to separate second distillate in line 145.This may allow a portion of either the resulting acetaldehyde-containingstream or ethyl acetate-containing stream to be recycled to reactor 103while purging the other stream. The purge stream may be valuable as asource of either ethyl acetate and/or acetaldehyde.

In one embodiment, it may be preferred to operate second column 138 inFIG. 2 at a pressure less than atmospheric pressure to decrease theenergy required to separate ethyl acetate and ethanol. However, thedecreased pressure may not have the advantage of enhancing diethylacetal hydrolysis. FIG. 3 shows process similar to FIG. 2, except firstdistillate in line 137 and/or ethanol mixture stream 144 is split and afirst portion is fed via line 147 to a high pressure second column 148,and a second portion is fed via line 149 to a low pressure second column150. The relative amounts fed to the high pressure second column 148 andlow pressure second column 150 may be controlled based on theconcentration of diethyl acetal and/or ethyl acetate in the ethanolcontaining residue. When diethyl acetal concentrations in the ethanolcontaining residue increase a larger portion of first distillate in line137 and/or ethanol mixture stream 144 may be fed to line 147. Likewise,when ethyl acetate concentrations increase, a larger portion may be fedto low pressure second column 150 via line 149. A control valve may beused to regulate the flow between the first and second portions.

In FIG. 3, high pressure second column 148 operates at an increasedpressure above atmospheric pressure to yield a distillate comprisingethyl acetate, acetaldehyde, and diethyl acetal in line 151 and aresidue comprising ethanol in line 152. In one embodiment residue inline 152 contains low concentrations of diethyl acetal, e.g., less than1 wt. %, or less than 0.5 wt. %. High pressure second column 148 mayalso have an increased stripping section of at least 40 stages, e.g., atleast 50 stages or at least 60 stages.

Low pressure second column 150 operates at in a vacuum or a pressureless than atmospheric pressure, e.g., less than 70 kPa or less than 50kPa. In terms of second the low pressure second column 150 may operatefrom 0.1 to 100 kPa, e.g. from 0.1 to 70 kPa, or from 0.1 to 35 kPa. Thedecreased pressure may further enhance separation of ethyl acetate andethanol. Low pressure second column 150 also yield a distillatecomprising ethyl acetate, acetaldehyde, and diethyl acetal in line 153and a residue comprising ethanol in line 154. In one embodiment residuein line 154 contains low concentrations of ethyl acetate, e.g., lessthan 1 wt. %, or less than 0.5 wt. %.

The distillates of each column may be combined and returned to thereactor 103 in a similar manner as discussed above in FIG. 2. Inaddition, the residues of each column in FIG. 3, may be combined toproduce an ethanol product. In some embodiments, it may be desired toproduce separate ethanol products from each of the second columns 148and 150 in FIG. 3.

FIG. 3 illustrates high and low pressure columns in parallel. In someembodiments, it may be necessary to operate the high and low pressurecolumns in series. FIG. 4 illustrates a process where the firstdistillate in line 137 and/or ethanol mixture stream 144 is initiallyfed to the low pressure second column 150. As indicated above theresidue in line 154 may have low concentrations of ethyl acetate, butmay also have higher concentrations of diethyl acetal. The residue inline 154 is fed to a high pressure second column 148 to enhancehydrolysis of diethyl acetal. In high pressure second column 148,diethyl acetal is hydrolyzed to yield a distillate in line 151comprising acetaldehyde and a residue in line 152 comprising ethanol.Using the high pressure second column 148, at least 20% of the diethylacetal in residue in line 154 may be hydrolyzed, e.g., at least 30% orat least 50%. Thus, the concentration of diethyl acetal in residue inline 152 may be less than 1 wt. %, e.g., less than 0.5 wt. %.

FIG. 5 reverses the order of the low pressure second column 150 and highpressure second column 148 from FIG. 4. FIG. 5 illustrates a processwhere the first distillate in line 137 and/or ethanol mixture stream 144is initially fed to the high pressure second column 148. As indicatedabove the residue in line 152 may have low concentrations of diethylacetal, but may also have higher concentrations of ethyl acetate. Inhigh pressure second column 148, diethyl acetal is hydrolyzed to yield adistillate in line 151 and a residue in line 152 comprising ethanol. Thedistillate in line 151 is fed to a low pressure second column 150.Distillate in line 151 may comprise ethyl acetate and ethanol and it maybe desirable to recover the ethanol instead of recycling the ethanol toreactor 103. Using low pressure second column 150, any ethanol indistillate in line 151 is separated as the residue in line 154 and theethyl acetate is removed as the distillate in line 153. Distillate inline 153 is recycled to reactor 103. Residue in line 154 may be combinedwith residue in line 152 to form an ethanol product. In someembodiments, each of the residue streams may be used as separatedethanol products.

Returning to FIG. 2, pressure of the second column 138 may be changeddepending on the concentration of the diethyl acetal in the secondresidue in line 146. As the diethyl acetal concentration increases inthe second residue in line 146 above, for example, 1 wt. %, the pressureof second column 138 may also be increased. In addition, when ethylacetate concentration in the second residue in line 146 above, forexample, 1 wt. %, the pressure of second column 138 may be decreased.This allows second column 138 to be adjusted as necessary withoutadditional capital for a further distillation column.

In another embodiment, the liquid stream 112 from reaction zone 103 maybe separated used a process as shown in FIG. 6. In one preferredembodiment, reaction zone 101 of FIG. 6 operates at above 80% aceticacid conversion, e.g., above 90% conversion or above 99% conversion.Thus, the acetic acid concentration in the liquid stream 112 may be low.

In the exemplary embodiment shown in FIG. 6, liquid stream 112 isintroduced in the upper part of first column 160, e.g., upper half orupper third. In one embodiment, no entrainers are added to first column160. In first column 160, a weight majority of the ethanol, water,acetic acid, and other heavy components, if present, are removed fromliquid stream 112 and are withdrawn, preferably continuously, as residuein line 162. First column 160 also forms an overhead distillate, whichis withdrawn in line 161, and which may be condensed and refluxed, forexample, at a ratio of from 30:1 to 1:30, e.g., from 10:1 to 1:10 orfrom 1:5 to 5:1. The first distillate in line 161 preferably comprises aweight majority of the ethyl acetate from liquid line 112. In addition,distillate in line 161 may also comprise acetaldehyde.

In one embodiment, first column 160 may be operated at an increasedpressure to enhance diethyl acetal hydrolysis. In addition, first column160 may also have a stripping section that comprises at least 40 stages,e.g., at least 50 stages or at least 60 stages.

When column 160 is operated under about 170 kPa, the temperature of theresidue exiting in line 162 preferably is from 70° C. to 155° C., e.g.,from 90° C. to 130° C. or from 100° C. to 110° C. The base of column 160may be maintained at a relatively low temperature by withdrawing aresidue stream comprising ethanol, water, and acetic acid, therebyproviding an energy efficiency advantage. The temperature, at 170 kPa,of the distillate exiting in line 161 preferably is from 75° C. to 100°C., e.g., from 75° C. to 83° C. or from 81° C. to 84° C.

In some embodiments, when the second column 163 of FIG. 6 is operated atan increased pressure, the pressure of first column 170 may range from0.1 kPa to 510 kPa, e.g., from 1 kPa to 475 kPa or from 1 kPa to 375kPa.

In an embodiment of the present invention, column 160 of FIG. 6 may beoperated at a temperature where most of the water, ethanol, and aceticacid are removed from the residue stream and only a small amount ofethanol and water is collected in the distillate stream due to theformation of binary and tertiary azeotropes. The weight ratio of waterin the residue in line 162 to water in the distillate in line 161 may begreater than 1:1, e.g., greater than 2:1. The weight ratio of ethanol inthe residue to ethanol in the distillate may be greater than 1:1, e.g.,greater than 2:1.

The amount of acetic acid in the first residue may vary dependingprimarily on the conversion in reactor 103. In one embodiment, when theconversion is high, e.g., greater than 90%, the amount of acetic acid inthe first residue may be less than 10 wt. %, e.g., less than 5 wt. % orless than 2 wt. %. In other embodiments, when the conversion is lower,e.g., less than 90%, the amount of acetic acid in the first residue maybe greater than 10 wt. %.

The distillate preferably is substantially free of acetic acid, e.g.,comprising less than 1000 ppm, less than 500 ppm or less than 100 ppmacetic acid. The distillate may be purged from the system or recycled inwhole or part to reactor 103. In some embodiments, the distillate may befurther separated, e.g., in an optional fourth column of FIG. 1, into anacetaldehyde stream and an ethyl acetate stream. Either of these streamsmay be returned to the reactor 103 or separated from system 100 as aseparate product.

To recover ethanol, the residue in line 162 may be further separated ina second column 163, also referred to as an “acid separation column.” Anacid separation column may be used when the acetic acid concentration inthe first residue is greater than 1 wt. %, e.g., greater than 5 wt. %.The first residue in line 162 is introduced to second column 163preferably in the top part of column 163, e.g., top half or top third.Second column 163 yields a second residue in line 165 comprising aceticacid and water, and a second distillate in line 164 comprising ethanol.

Second column 163 may be a tray column or packed column. In oneembodiment, second column 163 is a tray column having from 5 to 150trays, e.g., from 15 to 50 trays or from 20 to 45 trays. Similar tofirst column 160, second column may have a stripping section of at least40 stages, e.g., at least 50 stages or at least 60 stages.

In one embodiment, second column 163 may be operated at an increasedpressure to enhance diethyl acetal hydrolysis. In FIG. 6, it is morepreferred to operate the first column 160 at an increased pressure,because second column 163 comprises very low amounts of acetaldehydeand/or acetals. Generally, the pressure of second column 163 may rangefrom 0.1 kPa to 510 kPa, e.g., from 1 kPa to 475 kPa or from 1 kPa to375 kPa. At atmospheric pressure the temperature of the second residueexiting in line 165 preferably is from 95° C. to 130° C., e.g., from100° C. to 125° C. or from 110° C. to 120° C. The temperature of thesecond distillate exiting in line 164 preferably is from 60° C. to 105°C., e.g., from 75° C. to 100° C. or from 80° C. to 100° C.

The weight ratio of ethanol in the second distillate in line 164 toethanol in the second residue in line 165 preferably is at least 35:1.In one embodiment, the weight ratio of water in the second residue 165to water in the second distillate 164 is greater than 2:1, e.g., greaterthan 4:1 or greater than 6:1. In addition, the weight ratio of aceticacid in the second residue 165 to acetic acid in the second distillate164 preferably is greater than 10:1, e.g., greater than 15:1 or greaterthan 20:1. Preferably, the second distillate in line 164 issubstantially free of acetic acid and may only contain, if any, traceamounts of acetic acid. Preferably, the second distillate in line 164contains substantially no ethyl acetate.

The remaining water from the second distillate in line 164 may beremoved in further embodiments of the present invention. Depending onthe water concentration, the ethanol product may be derived from thesecond distillate in line 164. Some applications, such as industrialethanol applications, may tolerate water in the ethanol product, whileother applications, such as fuel applications, may require an anhydrousethanol. The amount of water in the distillate of line 164 may be closerto the azeotropic amount of water, e.g., at least 4 wt. %, preferablyless than 20 wt. %, e.g., less than 12 wt. % or less than 7.5 wt. %.Water may be removed from the second distillate in line 164 usingseveral different separation techniques as described herein.Particularly preferred techniques include the use of distillationcolumn, membranes, adsorption units, and combinations thereof.

Some of the residues withdrawn from the separation zone 102 compriseacetic acid and water. Depending on the amount of water and acetic acidcontained in the residue of first column, for example first residue 120in FIG. 1, the residue may be treated in one or more of the followingprocesses. The following are exemplary processes for further treatingthe residue and it should be understood that any of the following may beused regardless of acetic acid concentration. When the residue comprisesa majority of acetic acid, e.g., greater than 70 wt. %, the residue maybe recycled to the reactor without any separation of the water. In oneembodiment, the residue may be separated into an acetic acid stream anda water stream when the residue comprises a majority of acetic acid,e.g., greater than 50 wt. %. Acetic acid may also be recovered in someembodiments from the residue having a lower acetic acid concentration.The residue may be separated into the acetic acid and water streams by adistillation column or one or more membranes. If a membrane or an arrayof membranes is employed to separate the acetic acid from the water, themembrane or array of membranes may be selected from any suitable acidresistant membrane that is capable of removing a permeate water stream.The resulting acetic acid stream optionally is returned to the reactor103. The resulting water stream may be used as an extractive agent or tohydrolyze an ester-containing stream in a hydrolysis unit.

In other embodiments, for example, where the residue comprises less than50 wt. % acetic acid, possible options include one or more of: (i)returning a portion of the residue to reactor 103, (ii) neutralizing theacetic acid, (iii) reacting the acetic acid with an alcohol, or (iv)disposing of the residue in a waste water treatment facility. It alsomay be possible to separate a residue comprising less than 50 wt. %acetic acid using a weak acid recovery distillation column to which asolvent (optionally acting as an azeotroping agent) may be added.Exemplary solvents that may be suitable for this purpose include ethylacetate, propyl acetate, isopropyl acetate, butyl acetate, vinylacetate, diisopropyl ether, carbon disulfide, tetrahydrofuran,isopropanol, ethanol, and C₃-C₁₂ alkanes. When neutralizing the aceticacid, it is preferred that the residue comprises less than 10 wt. %acetic acid. Acetic acid may be neutralized with any suitable alkali oralkaline earth metal base, such as sodium hydroxide or potassiumhydroxide. When reacting acetic acid with an alcohol, it is preferredthat the residue comprises less than 50 wt. % acetic acid. The alcoholmay be any suitable alcohol, such as methanol, ethanol, propanol,butanol, or mixtures thereof. The reaction forms an ester that may beintegrated with other systems, such as carbonylation production or anester production process. Preferably, the alcohol comprises ethanol andthe resulting ester comprises ethyl acetate. Optionally, the resultingester may be fed to the hydrogenation reactor.

In some embodiments, when the residue comprises very minor amounts ofacetic acid, e.g., less than 5 wt. %, the residue may be disposed of toa waste water treatment facility without further processing. The organiccontent, e.g., acetic acid content, of the residue beneficially may besuitable to feed microorganisms used in a waste water treatmentfacility.

The columns shown in figures may comprise any distillation columncapable of performing the desired separation and/or purification. Forexample, other than the acid columns describe above, the other columnspreferably are a tray column having from 1 to 150 trays, e.g., from 10to 100 trays, from 20 to 95 trays or from 30 to 75 trays. The trays maybe sieve trays, fixed valve trays, movable valve trays, or any othersuitable design known in the art. In other embodiments, a packed columnmay be used. For packed columns, structured packing or random packingmay be employed. The trays or packing may be arranged in one continuouscolumn or they may be arranged in two or more columns such that thevapor from the first section enters the second section while the liquidfrom the second section enters the first section, etc.

The associated condensers and liquid separation vessels that may beemployed with each of the distillation columns may be of anyconventional design and are simplified in the figures. Heat may besupplied to the base of each column or to a circulating bottom streamthrough a heat exchanger or reboiler. Other types of reboilers, such asinternal reboilers, may also be used. The heat that is provided to thereboilers may be derived from any heat generated during the process thatis integrated with the reboilers or from an external source such asanother heat generating chemical process or a boiler. Although onereactor and one flasher are shown in the figures, additional reactors,flashers, condensers, heating elements, and other components may be usedin various embodiments of the present invention. As will be recognizedby those skilled in the art, various condensers, pumps, compressors,reboilers, drums, valves, connectors, separation vessels, etc., normallyemployed in carrying out chemical processes may also be combined andemployed in the processes of the present invention.

The temperatures and pressures employed in the columns may vary.Temperatures within the various zones will normally range between theboiling points of the composition removed as the distillate and thecomposition removed as the residue. As will be recognized by thoseskilled in the art, the temperature at a given location in an operatingdistillation column is dependent on the composition of the material atthat location and the pressure of column. In addition, feed rates mayvary depending on the size of the production process and, if described,may be generically referred to in terms of feed weight ratios.

The final ethanol product produced by the processes of the presentinvention may be taken from a stream that primarily comprises ethanolfrom exemplary systems shown in the FIGURES. The ethanol product may bean industrial grade ethanol comprising from 75 to 96 wt. % ethanol,e.g., from 80 to 96 wt. % or from 85 to 96 wt. % ethanol, based on thetotal weight of the ethanol product. Exemplary finished ethanolcompositional ranges are provided below in Table 3.

TABLE 3 FINISHED ETHANOL COMPOSITIONS Component Conc. (wt. %) Conc. (wt.%) Conc. (wt. %) Ethanol 75 to 99.9  80 to 99.5 85 to 96 Water <12 1 to9 3 to 8 Acetic Acid <1 <0.1 <0.01 Ethyl Acetate <2 <0.5 <0.05 DiethylAcetal <1 0.0001 to 0.1   0.0001 to 0.01  Acetone <0.05  <0.01  <0.005Isopropanol <0.5 <0.1 <0.05 n-propanol <0.5 <0.1 <0.05

The finished ethanol composition of the present invention preferablycontains very low amounts, e.g., less than 0.5 wt. %, of other alcohols,such as methanol, butanol, isobutanol, isoamyl alcohol and other C₄-C₂₀alcohols. In one embodiment, the amount of isopropanol in the finishedethanol composition is from 80 to 1,000 wppm, e.g., from 95 to 1,000wppm, from 100 to 700 wppm, or from 150 to 500 wppm. In one embodiment,the finished ethanol composition is substantially free of acetaldehyde,optionally comprising less than 8 wppm acetaldehyde, e.g., less than 5wppm or less than 1 wppm.

In some embodiments, when further water separation is used, the ethanolproduct may be withdrawn as a stream from the water separation unit asdiscussed above. In such embodiments, the ethanol concentration of theethanol product may be greater than indicated in Table 3, and preferablyis greater than 97 wt. % ethanol, e.g., greater than 98 wt. % or greaterthan 99.5 wt. %. The ethanol product in this aspect preferably comprisesless than 3 wt. % water, e.g., less than 2 wt. % or less than 0.5 wt. %.

The finished ethanol composition produced by the embodiments of thepresent invention may be used in a variety of applications includingfuels, solvents, chemical feedstocks, pharmaceutical products,cleansers, sanitizers, hydrogenation transport or consumption. In fuelapplications, the finished ethanol composition may be blended withgasoline for motor vehicles such as automobiles, boats and small pistonengine aircraft. In non-fuel applications, the finished ethanolcomposition may be used as a solvent for toiletry and cosmeticpreparations, detergents, disinfectants, coatings, inks, andpharmaceuticals. The finished ethanol composition may also be used as aprocessing solvent in manufacturing processes for medicinal products,food preparations, dyes, photochemicals and latex processing.

The finished ethanol composition may also be used as a chemicalfeedstock to make other chemicals such as vinegar, ethyl acrylate, ethylacetate, ethylene, glycol ethers, ethylamines, ethyl benzene, aldehydes,butadiene, and higher alcohols, especially butanol. In the production ofethyl acetate, the finished ethanol composition may be esterified withacetic acid. In another application, the finished ethanol compositionmay be dehydrated to produce ethylene. Any known dehydration catalystcan be employed to dehydrate ethanol, such as those described incopending U.S. Pub. Nos. 2010/0030002 and 2010/0030001, the entirecontents and disclosures of which are hereby incorporated by reference.A zeolite catalyst, for example, may be employed as the dehydrationcatalyst. Preferably, the zeolite has a pore diameter of at least about0.6 nm, and preferred zeolites include dehydration catalysts selectedfrom the group consisting of mordenites, ZSM-5, a zeolite X and azeolite Y. Zeolite X is described, for example, in U.S. Pat. No.2,882,244 and zeolite Y in U.S. Pat. No. 3,130,007, the entireties ofwhich are hereby incorporated herein by reference.

While the invention has been described in detail, modifications withinthe spirit and scope of the invention will be readily apparent to thoseof skill in the art. In view of the foregoing discussion, relevantknowledge in the art and references discussed above in connection withthe Background and Detailed Description, the disclosures of which areall incorporated herein by reference. In addition, it should beunderstood that aspects of the invention and portions of variousembodiments and various features recited below and/or in the appendedclaims may be combined or interchanged either in whole or in part. Inthe foregoing descriptions of the various embodiments, those embodimentswhich refer to another embodiment may be appropriately combined withother embodiments as will be appreciated by one of skill in the art.Furthermore, those of ordinary skill in the art will appreciate that theforegoing description is by way of example only, and is not intended tolimit the invention.

What is claimed is:
 1. A method for producing ethanol comprising:hydrogenating alkanoic acid and/or esters thereof in a reactor in thepresence of a catalyst to form a crude ethanol mixture comprisingacetaldehyde, ethanol, water, and diethyl acetal; and separating thecrude ethanol mixture in the one or more columns to recover an ethanolproduct comprises less than 1 wt. % diethyl acetal, wherein at least onecolumn is operated above atmospheric pressure.
 2. The method of claim 1,wherein the at least one column is operated at a pressure from 100 kPato 5,000 kPa.
 3. The method of claim 1, wherein ethanol is recovered inthe residue of the at least one column operated above atmosphericpressure.
 4. The method of claim 1, wherein the ethanol productcomprises from 0.0001 to 0.01 wt. % diethyl acetal.
 5. The method ofclaim 1, further comprising decomposing at least 10 to 75% of thediethyl acetal in the at least one column is operated above atmosphericpressure.
 6. A method for producing ethanol comprising: hydrogenatingalkanoic acid and/or esters thereof in a reactor in the presence of acatalyst to form a crude ethanol mixture; separating a portion of thecrude ethanol mixture in a first distillation column to yield a firstresidue comprising alkanoic acid, and a first distillate comprisingethanol, and acetaldehyde; and separating a portion of the firstdistillate in a second distillation column operated above atmosphericpressure to yield a second residue comprising ethanol, and a seconddistillate comprising acetaldehyde, wherein the second residue comprisesless than 1 wt. % diethyl acetal.
 7. The method of claim 6, wherein thesecond distillation column is operated at a pressure from 100 kPa to5,000 kPa.
 8. The method of claim 6, wherein the second distillationcolumn is operated at a pressure higher than the first column.
 9. Themethod of claim 6, wherein the second residue comprises from 0.0001 to0.01 wt. % diethyl acetal.
 10. The method of claim 6, further comprisingreturning a portion of the first residue to the reactor.
 11. The methodof claim 6, further comprising returning a portion of the seconddistillate to the reactor.
 12. The method of claim 6, wherein thealkanoic acid is formed from methanol and carbon monoxide, wherein eachof the methanol, the carbon monoxide, and hydrogen for the hydrogenatingstep is derived from syngas, and wherein the syngas is derived from acarbon source selected from the group consisting of natural gas, oil,petroleum, coal, biomass, and combinations thereof.
 13. A method forproducing ethanol comprising: providing a crude ethanol mixturecomprising acetic acid, acetaldehyde, ethanol, water, and diethylacetal; separating a portion of the crude ethanol mixture in a firstdistillation column to yield a first residue comprising acetic acid, anda first distillate comprising ethanol, and acetaldehyde; and separatinga portion of the first distillate in a second distillation columnoperated above atmospheric pressure to yield a second residue comprisingethanol, and a second distillate comprising acetaldehyde, wherein thesecond residue comprises less than 1 wt. % diethyl acetal.
 14. A methodfor producing ethanol comprising: hydrogenating alkanoic acid and/oresters thereof in a reactor in the presence of a catalyst to form acrude ethanol mixture; separating a portion of the crude ethanol mixturein a first distillation column to yield a first residue comprisingalkanoic acid, and a first distillate comprising ethanol, water, ethylacetate, and acetaldehyde; separating a portion of the first distillatein a second distillation column operated above atmospheric pressure toyield a second residue comprising ethanol and water, and a seconddistillate comprising ethyl acetate and acetaldehyde; and separating aportion of the second residue in a third distillation column operated toyield a third residue comprising water, and a third distillatecomprising ethanol, wherein the third distillate comprises less than 1wt. % diethyl acetal.
 15. The method of claim 14, wherein the seconddistillation column is operated at a pressure from 100 kPa to 5,000 kPa.16. The method of claim 14, wherein the second distillation column isoperated at a pressure higher than the first column.
 17. The method ofclaim 14, wherein the third distillate comprises from 0.0001 to 0.01 wt.% diethyl acetal.
 18. The method of claim 14, further comprisingreturning a portion of the first residue to the reactor.
 19. The methodof claim 14, further comprising returning a portion of the seconddistillate to the reactor.
 20. The method of claim 14, furthercomprising returning a portion of the third residue to the seconddistillation column.
 21. A method for producing ethanol comprising:providing a crude ethanol mixture comprising acetic acid, acetaldehyde,ethanol, water, and diethyl acetal; separating a portion of the crudeethanol mixture in a first distillation column to yield a first residuecomprising acetic acid, and a first distillate comprising ethanol,water, ethyl acetate, and acetaldehyde; separating a portion of thefirst distillate in a second distillation column operated aboveatmospheric pressure to yield a second residue comprising ethanol andwater, and a second distillate comprising ethyl acetate andacetaldehyde; and separating a portion of the second residue in a thirddistillation column operated to yield a third residue comprising water,and a third distillate comprising ethanol, wherein the third distillatecomprises less than 1 wt. % diethyl acetal.